The technology of time domain reflectometry has been used in measuring liquid levels in a tank. Particularly where closed containers are utilized, time domain reflectometry (TDR) has allowed for remote liquid level measurement. TDR uses an electronic signal sent along a "transmission line" which is reflected at any change in dielectric constant or impedance interface, i.e., air and water. Time domain reflectometry is set forth in TIME DOMAIN REFLECTOMETRY FOR LIQUID LEVEL MEASUREMENTS by George D. Lehmkuhl, Dow Chemical U.S.A., RFP-1902, Aug. 21, 1972, and also in CONTINUOUS LIQUID LEVEL MEASUREMENTS WITH TIME-DOMAIN REFLECTOMETRY by J. E. Cruz et al, Advances in Cryogenic Engineering, 1973. In general, either a step function or pulse may be sent down the transmission line. When a change in impedance of the line is encountered by the transmitted signal, a reflection occurs. This reflection can be detected, and the time interval between the transmitted wave and the reflection can be used as a measure of the distance to the impedance change or interface. This time interval is used to measure the depth of liquid in a tank. Such coaxial probes have generally included an outer stainless steel tube and an inner stainless tube, both welded to a bottom plate, and having apertures included in the outer stainless steel tube to allow the fluid to flow therein.
U.S. Pat. No. 5,136,249, to White et al discloses probes for measuring level for use in TDR, and discloses the use of one, three and four probes of various spacing. However, this reference does not teach a sensor which has the ability to change the spacing of the probe within the same bracket or while connected in the tank.
U.S. Pat. No. 5,103,672, to Terry, Jr. et al discloses a flexible transmitter and fluid level engaging probe which includes spacers about the electrode within the tube to maintain proper spacing therein. However, this patent relates to a capacitive-type probe and not a true TDR probe.
The problems with the prior art probes are that generally tanks are of various contours and it is beneficial to utilize a flexible probe, and furthermore, various fluid or substance types may be utilized and the spacing of the coaxial sensor should be varied depending on the substance.